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Zhigiang Zhu and Paul G. Thompson

The polymorphisms of phosphoglucose isomerase (PGI) in sweetpotato and I. trifida were examined. Horizontal starch gel electrophoresis was used to analyze leaf and pollen tissue of parents and progenies of 10 crosses. Analyses revealed that PGI was a dimeric enzyme system controlled by 5 loci. The segregation ratios did not suggest that PGI was a duplicate system and therefore did not indicate hexaploidy. Only 2 loci appeared to be present in I. trifida. No observed band was related to different ploidy levels in I. batatas and I. trifida. No linkage was identified among the loci.

Kittipat Ukoskit and Paul G. Thompson

Low-density randomly amplified polymorphic DNA (RAPD) markers of sweetpotato [Ipomoea batatus (L.) Lam.; 2n = 6x = 90] were constructed from 76 pseudotestcross progenies obtained from `Vardaman' × `Regal'. Of 460 primers, 84 generating 196 well-resolved repeatable markers were selected for genetic analysis. `Vardaman' and `Regal' testcross progenies were analyzed for segregation and linkages of RAPD markers. Type of polyploidy, autopolyploidy, or allopolyploidy is uncertain in sweetpotato and was examined in this study using the ratio of nonsimplex to simplex RAPD markers and the ratio of simplex RAPD marker pairs linked in repulsion to coupling. Both measures indicated autopolyploidy. Low-density RAPD linkage maps of `Vardaman' and `Regal' were constructed from simplex RAPD marker linkage analysis. Duplex and triplex markers were then mapped manually into the simplex marker map. Homologous linkage groups were identified using nonsimplex RAPD markers and three homologous groups were found in each of the parent maps. Use of nonsimplex markers increased mapping efficiency. The `Vardaman' map had a predicted coverage of 10.5% at a 25-cM interval of the genome size of 5024 cM. In `Regal', genome coverage was estimated to be 5.6% at a 25-cM interval of the genome size of 6560 cM. Therefore, average chromosome length was ≈56 to 73 cM.

Salvador Rojas and Paul G. Thompson

To develop In vitro techniques to overcome incompatibility in sweetpotato the cross and self incompatible cultivars Regal and MD-708 were cross pollinated and also crossed with the compatible `Vardaman'. Observation of pollen behavior in different crosses after 3, 7, and 24 hours, showed good germination and tube development in compatible crosses, but no germination in incompatibles. In a preliminary experiment using embryo rescue techniques plants were produced only from compatible crosses at 25 and 30 days after pollination. In subsequent experiments, immature embryos were rescued when cultured 15 days after pollination. The highest percentage of rescued embryos resulted from Murashige-Skoog medium. Intraovarian, stigmatic and placental in vitro fertilization were investigated to overcome incompatibility. Embryos were not formed from any of those methods, but callus was produced with placental pollination.

Jesse R. Quarrels and Paul G. Thompson

An experiment was conducted to determine the rate and frequency of irrigation needed for optimum yield in sweetpotato (Ipomoeabatatas (L.)Lam). A line source irrigation system was used to provide continuously increasing amounts of water at each irrigation. The physiological responses of sweetpotato to water application were measured. There was an increase in leaf water potential with increasing rates of irrigation. Leaf diffusive resistance decreased as total water rate increased to 76% of pan evaporation (Epan) and then increased with higher rates of irrigation. Marketable yields increased as total water rate increased to 76% of Epan and then decreased rapidly with higher irrigation rates. Water relations measurements indicated that reduction in yield with higher amounts of water application was due to low soil oxygen content.

Jeff L. Main and Paul G. Thompson

Three treatments were used to evaluate the effects of missing plants on sweetpotato yield: a single plant missing, two adjacent plants missing and two plants missing separated by a single plant. Individual plant yields of the four plants in the same row in each direction from the missing hill were taken. Yields were also taken from corresponding plants in the rows on each side of the row with the missing plant. A plot therefore consisted of three rows with the center row containing eight or nine plants and one or two missing hills and the other two rows containing nine to 12 plants. A single missing plant tended to increase yield of all grades of the plant in the same row next to the missing hill, but differences were nonsignificant. Two missing plants did not result in individual plant differences, but did increase overall plot yield of jumbo and cull grades. The single plant between two missing hills produced a greater number of small-sized No. 1 roots. No. 1 yield of plants in adjacent rows across from the single plant produced lower No. 1 yields.

Liang L. Hong and Paul G. Thompson

Random amplified polymorphic DNA (RAPD) markers were analyzed in parents and progeny of four sweetpotato crosses. An average of 69 primers were tested and 23.5% produced well resolved polymorphic banding patterns. Each polymorphic primer had an average of 1.9 polymorphic bands resulting in 0.45 polymorphic fragments per primer tested. Phenotypic segregation ratios of 88% of polymorphic fragments fit those expected for hexaploid Mendelian inheritance. Numbers of linked polymorphic fragments and numbers of linkage groups were 13 and 5 for Cross A, 0 and 0 for Cross B, 23 and 3 for Cross C and 16 and 6 for Cross D. Those results indicated that RAPD markers have potential for a genetic linkage map in sweetpotato; however, many primers must be screened.

Paul G. Thompson, John Schneider and Boyett Graves

A breeding population including parents with known moderate levels of weevil resistance was evaluated for resistance in a field experiment with few naturally occurring weevils. Weevils were collected from several Mississippi locations and intermated for increase. One male and one female were applied to the crown of each plant 73 days after transplanting. Seventeen parents and their progenies plus 6 additional clones or families were grown in 5 plant plots replicated 8 times. Genetic variability for resistance to weevil injury was observed. Mean percentage injured roots ranged from 25 to 85 and there were differences in resistance to injury among genotypes. Additive genetic variance was moderate so increased resistance levels should result from selection and intermating for high resistance.

Paul G. Thompson, Doyle A. Smittle and Melvin R. Hall

A line-source irrigation design was used to provide continuously increasing amounts of irrigation at each application to sweetpotato [Ipomoea batatas (L.) Lam]. Marketable yields increased with applied irrigation amounts until a total water application of 76% of pan evaporation (Epan) was reached and then decreased rapidly with applied irrigation amounts. Weight loss and decay of roots during storage showed quadratic responses to irrigation amounts and were minimal at the irrigation level of maximum yields. Contents of dextrins and maltose increased with irrigation amounts. Glucose content was maximum at a total water amount of 94% Epan and fructose content decreased with increased amounts of irrigation. Sensory ratings for appearance, flavor, texture, and preference, and objective color measurements of cooked flesh also reached their highest values near the irrigation amount of maximum yield.